Another interpretation: they said, now they are coming to stir up the quarrel that was between our father and Abraham, as it is said, "And there was strife between the herdsmen of Abram's livestock" (Genesis 13:7), and so forth.
Moab Fears the Old Quarrel of Lot and Abraham
Yalkut Shimoni on Torah 251:13
דָּבָר אַחֵר אָמְרוּ, עַכְשָׁיו הֵן בָּאִין לְעוֹרֵר מְרִיבָה שֶׁבֵּין אָבִינוּ לְאַבְרָהָם, שֶׁנֶּאֱמַר "וַיְהִי רִיב בֵּין רֹעֵי מִקְנֵה אַבְרָם" וְגוֹ'.